This invention relates to a charging control apparatus and more particularly to such apparatus which appropriately controls the charging of various storage batteries, inclusive of rapid charging of enclosed nickel-cadmium batteries.
There are various known apparatus of such type. For example, a device which stores the highest value of the terminal voltage of a storage battery and controls the charging when the terminal voltage is lowered by more than a predetermined value from the highest value. According to this device a fully charged state of an enclosed nickel-cadmium battery is achieved by controlling the charging when the terminal voltage is lowered by a predetermined value from its highest value at the end of the charging. Such predetermined value is about 20 mV at 1 C charging and a very fine value. In addition, in this charging device, the highest terminal voltage stored in the capacitor decreases with time due to leakage of electric current, so that control of the charging becomes impossible.
When the battery is overcharged, overdischarged or left unused for a long time, a peak value of the terminal voltage such as generated at the charging end may be generated at the beginning of the charging, as shown in FIG. 3. When the environmental temperature is above 40.degree. C., the terminal voltage may virtually not decrease even at the end of the charging, as shown in FIG. 4, to thereby generate insufficient charging or overcharging.
In order to solve such problems, there is a known device which has either an initial lock timer which stops the charging control for a predetermined time at the beginning of the charging or an overcharging protective timer which stops the charging a predetermined time after the charging has started. Provision of such timer would render the circuit complicated. In addition, if a partially discharged battery which, for example, has a remaining capacity of about 50% is 1C charged under a condition where the environmental temperature is above 40.degree. C., a peak value will be generated in about 30 minutes. However, since the terminal voltage hardly lowers, a terminal voltage which is lowered by the predetermined value from the highest value cannot be sensed, so that the charging continues until a time set by the overcharging protective timer to thereby create overcharging.
This invention eliminates the above drawbacks. It is an object of this invention to provide a charging control apparatus which has a circuit structure capable of appropriately charging any kind of storage battery in any discharged state to thereby to prevent overcharging and insufficient charging of various kinds of storage batteries including an enclosed battery used in electronic devices, especially partially discharged batteries.